Fatigue crack resistant nickel base superalloys and product formed

ABSTRACT

The present invention provides an alloy having improved crack growth inhibition and having high strength at high temperature. The composition of the alloy is essentially as follows: 
     
         ______________________________________                                    
 
    
                Concentration in Weight %                                      
           Claimed Composition                                            
Ingredient   From       To                                                
______________________________________                                    
Ni           balance                                                      
Co           4          12                                                
Cr           7          13                                                
Mo           2          6                                                 
Al           3.0        6.0                                               
Ti           3.5        5.0                                               
Ta           2.0        4.0                                               
Nb           1.0        3.0                                               
Re           0.0        3.0                                               
Hf           0.0        0.75                                              
Zr           0.0        0.10                                              
V            0.0        3.0                                               
C            0.0        0.20                                              
B            0.0        0.10                                              
W            0.0        1.0                                               
Y            0.0        0.10                                              
______________________________________

RELATED APPLICATIONS

The subject application relates generally to the subject matter ofapplication Ser. No. 907,550, filed Sept. 15, 1986 as well as to Ser.No. 080,353, filed July 31, 1987. It also relates to Ser. Nos. 103,851;103,906 and 104,001, filed Oct. 2, 1987. Further, it relates to Ser. No.250,204, filed Aug. 28, 1988; Ser. No. 248,756, filed Sept. 26, 1988;Ser. No. 250,205, filed Sept. 28, 1988; Ser. No. 248,755, filed Sept.26, 1988; and to Ser. No. 248,754, filed Sept. 26, 1988. The texts ofthe related applications and of the applications referenced therein areincorporated herein by reference.

BACKGROUND OF THE INVENTION

It is well known that nickel based superalloys are extensively employedin high performance environments. Such alloys have been used extensivelyin jet engines, in land based gas turbines and other machinery wherethey must retain high strength and other desirable physical propertiesat elevated temperatures of a 1000° F. or more.

Many of these alloys contain a γ' precipitate in varying volumepercentages. The γ' precipitate contributes to the high performanceproperties of such alloys at their elevated use temperatures.

More detailed characteristics of the phase chemistry of γ' are given in"Phase Chemistries in Precipitation-Strengthening Superalloy" by E. L.Hall, Y. M. Kouh, and K. M. Chang [Proceedings of 41st Annual Meeting ofElectron Microscopy Society of America, August 1983 (p. 248)].

The following U.S. patents disclose various nickel-base alloycompositions: U.S. Pat No. 2,570,193; U.S. Pat. No. 2,621,122; U.S. Pat.No. 3,046,108; U.S. Pat. No. 3,061,426; U.S. Pat. No. 3,151,981; U.S.Pat. No. 3,166,412; U.S. Pat. No. 3,322,534; U.S. Pat. No. 3,343,950;U.S. Pat. No. 3,575,734; U.S. Pat. No. 3,576,861; U.S. Pat. No.4,207,098 and U.S. Pat. No. 4,336,312. The aforementioned patents arerepresentative of the many alloying developments reported to date inwhich many of the same elements are combined to achieve distinctlydifferent functional relationships between the elements such that phasesproviding the alloy system with different physical and mechanicalcharacteristics are formed. Nevertheless, despite the large amount ofdata available concerning the nickel-base alloys, it is still notpossible for workers in the art to predict with any significant degreeof accuracy the physical and mechanical properties that will bedisplayed by certain concentrations of known elements used incombination to form such alloys even though such combination may fallwithin broad, generalized teachings in the art, particularly when thealloys are processed using heat treatments different from thosepreviously employed.

A problem which has been recognized to a greater and greater degree withmany such nickel based superalloys is that they are subject to formationof cracks or incipient cracks, either in fabrication or in use, and thatthe cracks can actually propagate or grow while under stress as duringuse of the alloys in such structures as gas turbines and jet engines.The propagation or enlargement of cracks can lead to part fracture orother failure. The consequence of the failure of the moving mechanicalpart due to crack formation and propagation is well understood. In jetengines it can be particularly hazardous.

U.S. Pat. No. 4,685,977, entitled "Fatigue-Resistant Nickel-BaseSuperalloy and Method" is assigned to the same assignee as the subjectapplication. It discloses an alloy having a superior resistance tofatigue crack propagation based on alloy chemistry, γ' precipitatecontent and grain structure. A method of alloy preparation is alsotaught.

However, what has been poorly understood until recent studies wereconducted was that the formation and the propagation of cracks instructures formed of superalloys is not a monolithic phenomena in whichall cracks are formed and propagated by the same mechanism and at thesame rate and according to the same criteria. By contrast the complexityof the crack generation and propagation and of the crack phenomenagenerally and the interdependence of such propagation with the manner inwhich stress is applied is a subject on which important new informationhas been gathered in recent years. The variability from alloy to alloyof the effect of the period during which stress is applied to a memberto develop or propagate a crack, the intensity of the stress applied,the rate of application and of removal of stress to and from the memberand the schedule of this application was not well understood in theindustry until a study was conducted under contract to the NationalAeronautics and Space Administration. This study is reported in atechnical report identified as NASA CR-165123 issued from the NationalAeronautics and Space Administration in August 1980, identified as"Evaluation of the Cyclic Behavior of Aircraft Turbine Disk Alloys" PartII, Final Report, by B. A. Cowles, J. R. Warren and F. K. Hauke, andprepared for the National Aeronautics and Space Administration, NASALewis Research Center, Contract NAS3-21379.

A principal finding of the NASA sponsored study was that the rate ofpropagation based on fatigue phenomena or in other words, the rate offatigue crack propagation (FCP), was not uniform for all stressesapplied nor to all manners of applications of stress. More importantly,the finding was that fatigue crack propagation actually varied with thefrequency of the application of stress to the member where the stresswas applied in a manner to enlarge the crack. More surprising still, wasthe magnitude of the finding from the NASA sponsored study that theapplication of stress of lower frequencies rather than at the higherfrequencies previously employed in studies, actually increased the rateof crack propagation. In other words the NASA study verified that therewas a time dependence in fatigue crack propagation. Further the timedependence of fatigue crack propagation was found to depend not onfrequency alone but on the time during which the member was held understress or a so-called hold-time.

Following the documentation of this unusual degree of increased fatiguecrack propagation at lower stress frequencies there was some belief inthe industry that this newly discovered phenomena represented anultimate limitation on the ability of the nickel based superalloys to beemployed in the stress bearing parts of the turbines and aircraftengines and that all design effort had to be made to design around thisproblem.

However, it has been discovered that it is feasible to construct partsof nickel based superalloys for use at high stress in turbines andaircraft engines with greatly reduced crack propagation rates and withgood high temperature strength.

It is known that the most demanding sets of properties for superalloysare those which are needed in connection with jet engine construction.Of the sets of properties which are needed those which are needed forthe moving parts of the engine are usually greater than those needed forstatic parts, although the sets of needed properties are different forthe different components of an engine.

Because some sets of properties are not attainable in cast alloymaterials, resort is sometimes had to the preparation of parts by powdermetallurgy techniques. However, one of the limitations which attends theuse of powder metallurgy techniques in preparing moving parts for jetengines is that of the purity of the powder. If the powder containsimpurities such as a speck of ceramic or oxide the place where thatspeck occurs in the moving part becomes a latent weak spot where a crackmay initiate. Such a weak spot is in essence a latent crack. Thepossible presence of such latent cracks makes the problems of reducingand inhibiting the crack propagation rate all the more important. I havefound that it is possible to inhibit crack propagation both by thecontrol of the composition of alloys and by the methods of preparationof such metal alloys.

Pursuant to the present invention, a superalloy which can be prepared bypowder metallurgy techniques is provided. Also a method for processingthis superalloy to produce materials with a superior set or combinationof properties for use in advanced engine disk applications is provided.The properties which are conventionally needed for materials used indisk applications include high tensile strength and high stress rupturestrength. In addition the alloy of the subject invention exhibits adesirable property of resisting time dependent crack growth propagation.Such ability to resist crack growth is essential for the component LCFlife.

As alloy products for use in turbines and jet engines have developed ithas become apparent that different sets of properties are needed forparts which are employed in different parts of the engine or turbine.For jet engines the material requirements of more advanced aircraftengines continue to become more strict as the performance requirementsof the aircraft engines are increased. The different requirements areevidenced, for example, by the fact that many blade alloys display verygood high temperature properties in the cast form. However, the directconversion of cast blade alloys into disk alloys is very unlikelybecause blade alloys display inadequate strength at intermediatetemperatures. Further, the blade alloys have been found very difficultto forge and forging has been found desirable in the fabrication ofdisks from disk alloys. Moreover, the crack growth resistance of diskalloys has not been evaluated. Accordingly to achieve increased engineefficiency and greater performance, constant demands are made forimprovements in the strength and temperature capability of disk alloysas a special group of alloys for use in aircraft engines.

Accordingly what was sought in undertaking the work which lead to thepresent invention was the development of a disk alloy having a low orminimum time dependence of fatigue crack propagation and moreover a highresistance to fatigue cracking. In addition what was sought was abalance of properties and particularly of tensile, creep and fatigueproperties. Further what was sought was an enhancement of establishedalloy systems relative to inhibition of crack growth phenomena.

The development of the superalloy compositions and methods of theirprocessing of this invention focuses on the fatigue property andaddresses in particular the time dependence of crack growth.

Crack growth, i.e., the crack propagation rate, in high-strength alloybodies is known to depend upon the applied stress (σ) as well as thecrack length (a). These two factors are combined by fracture mechanicsto form one single crack growth driving force; namely, stress intensityfactor K, which is proportional to σ√a. Under the fatigue condition, thestress intensity in a fatigue cycle may consist of two components,cyclic and static. The former represents the maximum variation of cyclicstress intensity (ΔK), i.e., the difference between K_(max) and K_(min).At moderate temperatures, crack growth is determined primarily by thecyclic stress intensity (ΔK) until the static fracture toughness K_(IC)is reached. Crack growth rate is expressed mathematically as da/dN∝(ΔK)^(n). N represents the number of cycles and n is materialdependent. The cyclic frequency and the shape of the waveform are theimportant parameters determining the crack growth rate. For a givencyclic stress intensity, a slower cyclic frequency can result in afaster crack growth rate. This undesirable time-dependent behavior offatigue crack propagation can occur in most existing high strengthsuperalloys. To add to the complexity of this time-dependencephenomenon, when the temperature is increased above some point, thecrack can grow under static stress of some intensity K without anycyclic component being applied (i.e. ΔK=0). The design objective is tomake the value of da/dN as small and as free of time-dependency aspossible. Components of stress intensity can interact with each other insome temperature range such that crack growth becomes a function of bothcyclic and static stress intensities, i.e., both ΔK and K.

BRIEF DESCRIPTION OF THE INVENTION

It is, accordingly, one object of the present invention to providenickel-base superalloy products which are more resistant to cracking.

Another object is to provide a method for reducing the tendency of knownand established nickel-base superalloys to undergo cracking.

Another object is to provide articles for use under cyclic high stresswhich are more resistant to fatigue crack propagation.

Another object is to provide a composition and method which permitsnickel-base superalloys to have imparted thereto resistance to crackingunder stress which is applied cyclically over a range of frequencies.

Other objects will be in part apparent and in part pointed out in thedescription which follows.

In one of its broader aspects, objects of the invention can be achievedby providing a composition of the following approximate content:

    ______________________________________                                        Concentration in Weight %                                                     Claimed Composition                                                           Ingredient       From    To                                                   ______________________________________                                        Ni               balance                                                      Co               4       12                                                   Cr               7       13                                                   Mo               2       6                                                    Al               3.0     6.0                                                  Ti               3.5     5.0                                                  Ta               2.0     4.0                                                  Nb               1.0     3.0                                                  Zr               0.0     0.10                                                 V                0.0     3.0                                                  C                0.0     0.20                                                 B                0.0     0.10                                                 W                0.0     1.0                                                  ______________________________________                                    

In another of its broader aspects, objects of the invention can beachieved by providing a composition of the following approximatecontent:

    ______________________________________                                        Concentration in Weight %                                                     Claimed Composition                                                           Ingredient       From    To                                                   ______________________________________                                        Ni               balance                                                      Co               4       12                                                   Cr               7       13                                                   Mo               2       6                                                    Al               3.0     6.0                                                  Ti               3.5     5.0                                                  Ta               2.0     4.0                                                  Nb               1.0     3.0                                                  Re               0.0     3.0                                                  Hf               0.0     0.75                                                 Zr               0.0     0.10                                                 V                0.0     3.0                                                  C                0.0     0.20                                                 B                0.0     0.10                                                 W                0.0     1.0                                                  Y                0.0     0.10                                                 ______________________________________                                    

BRIEF DESCRIPTION OF THE DRAWINGS

In the description which follows clarity of understanding will be gainedby reference to the accompanying drawings in which:

FIG. 1 is a graph in which fatigue crack growth in inches per cycle isplotted on a log scale against ultimate tensile strength in ksi.

FIG. 2 is a plot similar to that of FIG. 1 but having an abscissa scaleof chromium content in weight %.

FIG. 3 is a plot of the log of crack growth rate against the hold timein seconds for a cyclic application of stress to a test specimen.

FIG. 4 is a graph in which the crack propagation rate, da/dN, in inchesper cycle is plotted against the cooling rate in degrees Farenheit perminute.

FIG. 5 is a graph of the yield stress in ksi at 750° F. plotted againstcooling rate in degrees Farenheit per minute on a log scale.

FIG. 6 is a graph of the ultimate tensile strength in ksi at 750° F.plotted against the cooling rate in degrees Farenheit per minute on alog scale.

FIG. 7 is a graph of the yield stress in ksi at 1400° F. plotted againstthe cooling rate in degrees Farenheit per minute.

FIG. 8 is a graph of the ultimate tensile strength in ksi at 1400° F.plotted against the cooling rate in degrees Farenheit per minute.

FIG. 9 is a second graph in which the crack propagation rate, da/dN, ininches per cycle is plotted against the cooling rate in degreesFarenheit per minute.

FIG. 10 is a second graph of the yield stress in ksi at 750° F. plottedagainst cooling rate in degrees Farenheit per minute on a log scale.

FIG. 11 is a second graph of the ultimate tensile strength in ksi at750° F. plotted against the cooling rate in degrees Farenheit per minuteon a log scale.

FIG. 12 is a second graph of the yield stress in ksi at 1400° F. plottedagainst the cooling rate in degrees Farenheit per minute.

FIG. 13 is a second graph of the ultimate tensile strength in ksi at1400° F. plotted against the cooling rate in degrees Farenheit perminute.

DETAILED DESCRIPTION OF THE INVENTION

I have discovered that by studying the present commercial alloysemployed in structures which required high strength at high temperaturethat the conventional superalloys fall into a pattern. This pattern isbased on plotting in a manner which I have devised of data appearing inthe Final Report NASA CR-165123 referenced above. I plotted the datafrom the NASA report of 1980 with the parameters arranged as indicatedin FIG. 1. There is a generally diagonally arrayed display of datapoints evident from a study of FIG. 1 of the drawings.

In FIG. 1, the crack growth rate in inches per cycle is plotted againstthe ultimate tensile strength in ksi. The individual alloys are markedon the graph by plus signs which identify the respective crack growthrate in inches per cycle characteristic of the alloy at an ultimatetensile strength in ksi which is correspondingly also characteristic forthe labeled alloy. As will be observed, a line identified as a 900second dwell time plot shows the characteristic relationship between thecrack growth rate and the ultimate tensile strength for theseconventional and well known alloys. Similar points corresponding tothose of the labeled pluses are shown at the bottom of the graph forcrack propagation rate tests conducted at 0.33 Hertz or in other words,at a higher frequency. A diamond data point appears in the region alongthe line labeled 0.33 Hertz for each labeled alloy shown in the upperpart of the graph.

From FIG. 1, it became evident that there is no alloy composition, whichhad coordinates of FIG. 1, which fell in the lower right hand corner ofthe graph for long dwell time. In fact, since all of the data points forthe longer dwell time crack growth testing fell along the diagonal lineof the graph, it appeared possible that any alloy composition which wasformed would fall somewhere along the diagonal line of the graph. Inother words, it appeared that it was possible that no alloy compositioncould be found which had both a high ultimate tensile strength and a lowcrack growth rate at long dwell times according to the parametersplotted in FIG. 1.

However, I have found that it is possible to produce an alloy which hasa composition which permits the unique combination of high ultimatestrength and low crack growth rate to be achieved.

One of the conclusions which I reached on a tentative basis regardingthe data plotted in FIG. 1 was that there may be some influence of thechromium concentration on the crack growth rate of the various alloys.For this reason, and using data from the 1980 NASA report, I plotted thechromium content in weight % against the crack growth rate and theresults of this plot is shown in FIG. 2. In this Figure, the chromiumcontent is seen to vary between about 9 to 19% and the correspondingcrack growth rate measurements indicate that as the chromium contentincreases, in general, the crack growth rate decreases. Based on thisgraph, it appeared that it might be very difficult or impossible todevise an alloy composition which had a low chromium content and alsohad a low crack growth rate at long dwell times.

However, I have found that it is possible through proper alloying of thecombined ingredients of a superalloy compositions to form a compositionwhich has both a low chromium content and a low crack growth rate atlong dwell times.

One way in which the relationship between the hold time for subjecting atest specimen to stress and the rate at which crack growth varies, isshown in FIG. 3. In this Figure, the log of the crack growth rate isplotted as the ordinate and the dwell time or hold time in seconds isplotted as the abscissa. A crack growth rate of 5×10⁻⁵ might be regardedas an ideal rate for cyclic stress intensity factors of 25 ksi/in. If anideal alloy were formed the alloy would have this rate for any hold timeduring which the crack or the specimen is subjected to stress. Such aphenomenon would be represented by the line (a) of FIG. 3 whichindicates that the crack growth rate is essentially independent of thehold or dwell time during which the specimen is subjected to stress.

By contrast a non-ideal crack growth rate but one which actuallyconforms more closely to the actual phenomena of cracking is shown inFIG. 3 by the line plotted as line (b). For very short hold time periodsof a second or a few seconds, it is seen that the ideal line (a) and thepractical line (b) are separated by a relatively small amount. At thesehigh frequencies, or low hold time, stressing of the sample the crackgrowth rate is relatively low.

However, as the hold time during which stress is applied to a sample isincreased, the results which are obtained from experiments forconventional alloys follow the line (b). Accordingly it will be seenthat there is an increase at greater than a linear rate as the frequencyof the stressing is decreased and the hold time for the stressing isincreased. At an arbitrarily selected hold time of about 500 seconds, itmay be seen from FIG. 3 that a crack growth rate may increase by twoorders of magnitude from 5×10⁻⁵ to 5×10⁻³ above the standard rate of5×10⁻⁵.

Again, it would be desirable to have a crack growth rate which isindependent of time and this would be represented ideally by the path ofthe line (a) as the hold time is increased and the frequency of stressapplication is decreased.

Remarkably, I have found that by making slight changes in theingredients of superalloys it is possible to greatly improve theresistance of the alloy to long dwell time crack growth propagation. Inother words it has been found possible to reduce the rate of crackgrowth by alloying modification of the alloys. Further increase can beobtained as well by the treatment of the alloy. Such treatment isprincipally a thermal treatment.

EXAMPLE 1

An alloy identified as HK104 was prepared. The composition of the alloywas essentially as follows:

    ______________________________________                                        Ingredient  Concentration in Weight %                                         ______________________________________                                        Ni          balance                                                           Co          8                                                                 Cr          10                                                                Mo          4                                                                 Al          4.5                                                               Ti          4.0                                                               Ta          3.0                                                               Nb          1.5                                                               Re          0.0                                                               Hf          0.0                                                               Zr          0.06                                                              V           1.0                                                               C           0.05                                                              B           0.03                                                              Y           0.0                                                               ______________________________________                                    

The alloy was subjected to various tests and the results of these testsare plotted in the FIGS. 4 through 8. Herein alloys are identified by anappendage "-SS" if the data that were taken on the alloy were taken onmaterial processed "super-solvus", i.e. the high temperature solid stateheat treatment given to the material was at a temperature above whichthe strengthening precipitate γ' dissolves and below the incipientmelting point. This usually results in grain size coarsening in thematerial. The strengthening phase γ' re-precipitates on subsequentcooling and aging.

Turning now to FIG. 4, the rate of crack propagation in inches per cycleis plotted against the cooling rate in °F. per minute. The samples ofRene' 95-SS and HK104-SS were tested in air at 1200° F. with a 1000second hold time at maximum stress intensity factor. As is evident, theHK104-SS has a lower crack growth rate than the Rene' 95-SS for samplescooled at all rates tried and that the HK104-SS cracks grow 4 to 20times slower. It should be noted that a range of cooling rates formanufactured components from such superalloys is expected to be in therange of 100° F./min to 600° F./min.

Regarding the other properties of the alloy, they are described herewith reference to the FIGS. 5, 6, 7 and 8.

The alloy of Example 1 is similar in certain respects to IN100 butcomparative testing of the subject alloy and samples of Rene' 95-SS werecarried out to provide a basis for comparing the subject alloy to analloy much stronger than IN100. Test results obtained at 750° F. areplotted in FIGS. 5 and 6 and test results obtained at 1400° F. areplotted in FIGS. 7 and 8.

Reference is made first to the test data plotted in FIG. 5. In FIG. 5,there is plotted a relationship between the yield stress in ksi and thecooling rate in °F. per minute for two alloy samples, HK104-SS and Rene'95-SS tests on which were performed at 750° F. In this plot there isevidence of that the HK104-SS alloy is only 10 to 16% lower in yieldstrength at 750° F. than R'95-SS, an alloy well-known for its highstrength.

The samples of HK104-SS and Rene' 95-SS were both prepared by powdermetallurgy techniques and are accordingly quite comparable to eachother.

In FIG. 6, a plot is set forth of ultimate tensile strength in ksiagainst the cooling rate in °F. per minute for a sample preparedaccording to the above example of alloy HK104-SS and also by way ofcomparison, a sample of Rene' 95-SS The samples tested were measured at750° F. It is well-known that Rene' 95 is one of the strongestcommercially availale superalloys which is known. From FIG. 6, it isevident that the ultimate tensile strength measurements made on therespective samples of the HK104-SS alloy and the Rene' 95-SS alloydemonstrated that the HK104-SS alloy indeed has ultimate tensilestrength which is essentially equivalent to the Rene' 95-SS material.

Turning now to FIGS. 7 and 8, there is plotted the relationship betweenthe yield strength and ultimate tensile at 1400° F. versus the coolingrate in °F. per minute for two alloys, one being Rene' 95-SS and theother being HK104-SS both of which samples were tested at 1400° F. Atmost HK104-SS is only 12% lower than Rene' 95-SS at higher cooling ratesand essentially equivalent to Rene' 95-SS at lower cooling rates foryield stress, and essentially equivalent to Rene' 95-SS for ultimatetensile strength.

Additionally, the ultimate tensile strength of 212 ksi measured at 1200°F. (649° C.) on material cooled at 360° F./min demonstrates a remarkableimprovement over the powder metallurgy IN100 of FIG. 1.

Moreover, with respect to inhibition of fatigue crack propagation thesubject alloys are far superior to Rene' 95 particularly those alloysprepared at cooling rates of 100° F./min to 600° F./min which are therates which are to be used for industrial production of the subjectalloy.

EXAMPLE 2

An alloy identified as HK103 was prepared. The composition of the alloywas essentially as follows:

    ______________________________________                                        Ingredient  Concentration in Weight %                                         ______________________________________                                        Ni          balance                                                           Co          8                                                                 Cr          10                                                                Mo          4                                                                 Al          4.8                                                               Ti          4.2                                                               Ta          3.0                                                               Nb          1.5                                                               Re          0.0                                                               Hf          0.0                                                               Zr          0.06                                                              V           0.0                                                               C           0.05                                                              B           0.03                                                              Y           0.0                                                               ______________________________________                                    

The alloy was subjected to various tests and the results of these testsare plotted in the FIGS. 9 through 13. As previously stated, alloys areidentified herein by an appendage "-SS" if the data that were taken onthe alloy were taken on material processed "super-solvus", i.e. the hightemperature solid state heat treatment given to the material was at atemperature above which the strengthening precipitate γ' dissolves andbelow the incipient melting point. The strengthening phase γ're-precipitates on subsequent cooling and aging.

Turning now to FIG. 9, the rate of crack propagation in inches per cycleis plotted against the cooling rate in °F. per minute. The samples ofRene' 95-SS and HK103-SS were tested in air at 1200° F. with a 1000second hold time at maximum stress intensity factor. As is evident, theHK103-SS has a lower crack growth rate than the Rene' 95-SS for samplescooled at all rates tried and that the HK103-SS cracks grow 8 to 60times slower. As noted above, a range of cooling rates for manufacturedcomponents from such superalloys is expected to be in the range of 100°F./min to 600° F./min.

Regarding the other properties of the subject alloy, they are describedhere with reference to the FIGS. 10, 11, 12 and 13.

The alloy of Example 2 is also similar in certain respects to IN100 andcomparative testing of the subject alloy and samples of Rene' 95-SS werecarried out to provide an additional basis for comparing the subjectalloy to an alloy much stronger than IN100. Test results obtained at750° F. are plotted in FIGS. 10 and 11 and test results obtained at1400° F. are plotted in FIGS. 12 and 13.

Reference is made first to the test data plotted in FIG. 10. In FIG. 10,there is plotted a relationship between the yield stress in ksi and thecooling rate in °F. per minute for two alloy samples, HK103-SS and Rene'95-SS, tests on which were performed at 750° F. In this plot, there isevidence that the HK103-SS alloy is only 12 to 14% lower in yieldstrength at 750° F. than Rene' 95-SS, an alloy well-known for its highstrength.

As in the case of the HK104-SS alloy, the samples of HK103-SS and Rene'95-SS were both prepared by powder metallurgy techniques and are,accordingly, quite comparable to each other.

In FIG. 11, a plot is set forth of ultimate tensile strength in ksiagainst the cooling rate in °F. per minute for a sample preparedaccording to the above example of alloy HK103-SS and also by way ofcomparison, a sample of Rene' 95-SS. The samples tested were measured at750° F. It is well-known that Rene' 95 is one of the strongestcommercially available superalloys which is known. From FIG. 11, it isevident that the ultimate tensile strength measurements made on therespective samples of the HK103-SS alloy and the Rene' 95-SS alloydemonstrated that the HK103-SS alloy indeed has ultimate tensilestrength which is closely comparable to the Rene' 95-SS material.

Turning now to FIGS. 12 and 13, there is plotted the relationshipbetween the yield strength and ultimate tensile at 1400° F. versus thecooling rate in °F. per minute for two alloys, one being Rene' 95-SS andthe other being HK103-SS, both of which samples were tested at 1400° F.At most, HK103-SS is only 16% lower than Rene' 95-SS at higher coolingrates and closely comparable to Rene' 95-SS at lower cooling rates foryield stress and only slightly below Rene' 95-SS for ultimate tensilestrength.

From the foregoing, it is evident that the invention provides alloyshaving unique combinations of ingredients based both on the ingredientidentification and on the relative concentrations thereof. It is alsoevident that the alloys which are proposed pursuant to the presentinvention have a novel and unique capability for crack propagationinhibition. The low crack propagation rates, da/dN, for the HK103-SS andHK104-SS alloys which are evident from FIGS. 4 and 9 is a uniquely noveland remarkable result.

This is quite surprising inasmuch as the constituents of the subjectalloys are only slightly different from constituents found in IN100alloy although the slight difference is critically important in yieldingdramatic differences, and specifically improvements in strength withoutan increase in crack propagation rates at long cycle fatigue tests. Itis this slight difference in ingredients and proportions which resultsin the surprising and unexpectedly low fatigue crack propagation ratescoupled with a highly desirable set of strength and other properties asalso evidenced from the graphs of the Figures of the subjectapplication.

What is remarkable about the achievement of the present invention is thestriking improvement which has been made in fatigue crack propagationresistance with a relatively small change in ingredients of the HK104and HK103 alloys as compared to those of the IN100 alloy.

To illustrate the small change in alloy compositions the ingredients ofthe IN100 and of both the HK104 and HK103 alloys are listed here.

                  TABLE I                                                         ______________________________________                                        Ingredient HK103        HK104   IN100                                         ______________________________________                                        Ni         64.36        63.86   60.68                                         Co         8            8       15                                            Cr         10           10      10                                            Mo         4            4       3.0                                           Al         4.8          4.5     5.5                                           Ti         4.2          4.0     4.7                                           Ta         3.0          3.0     --                                            Nb         1.5          1.5     --                                            Zr         0.06         0.06    0.06                                          C          0.05         0.05    0.01                                          B          0.03         0.03    0.01                                          V          0.0          1.0     1.0                                           ______________________________________                                    

From the above Table I, it is evident that the significant differencesbetween the composition of IN100 alloy as compared to that of alloyHK104 is that the subject alloy omits 7.0 weight percent cobalt, 1.0weight percent aluminum, and 0.70 weight percent titanium, and adds 3.0weight percent tantalum, 1.5 weight percent niobium and 1.0 weightpercent molybdenum.

With reference to the HK103 alloy, it differs from the HK104 alloy onlyin having a higher aluminum (4.8 vs. 4.5 for HK104), a higher titanium(4.2 vs. 4.0 for HK104) and lower vanadium (0.0 vs. 1.0 for HK104). Thecomparison between HK104 and IN100 applies to the HK103 alloy except inthese three respects.

It is deemed rather remarkable considering the teachings of FIG. 1 thatthis alteration of the composition can accomplish an increase orimprovement of the basic strength properties of the alloy almost up tothat of Rene' 95 and at the same time provide long dwell time fatiguecrack inhibition of the alloy. However, this is precisely the result ofthe alteration of the composition as is evidenced by the data which isgiven in the figures and discussed extensively above.

Other changes in ingredients may be made which do not cause suchremarkable change of properties, particularly smaller changes of someingredients. For example, small additions of rhenium may be made to theextent that they do not change, and parlticularly do not detract from,the uniquely beneficial combination of properties which have been foundfor the HK104 and HK103 alloys.

While the alloy is described above in terms of the ingredients andpercentages of ingredients which yield uniquely advantageousproportions, particularly with respect to inhibition of crackpropagation it will be realized that other ingredients such as yttrium,hafnium, etc., can be included in the composition in percentages whichdo not interfere with the novel crack propagation inhibition. A smallpercentage of yttrium between 0 and 0.1 percent may be included in thesubject alloy without detracting from the unique and valuablecombination of properties of the subject alloy.

What is claimed is:
 1. As a composition of matter an alloy consistingessentially of the following ingredients in the following proportions:

    ______________________________________                                                   Concentration in Weight %                                                     Claimed Composition                                                Ingredient   From       To                                                    ______________________________________                                        Ni           balance                                                          Co           4          12                                                    Cr           7          13                                                    Mo           2          6                                                     Al           3.0        6.0                                                   Ti           3.5        5.0                                                   Ta           2.0        4.0                                                   Nb           1.0        3.0                                                   Re           0.0        3.0                                                   Hf           0.0        0.75                                                  Zr           0.00       0.10                                                  V            0.0        3.0                                                   C            0.0        0.20                                                  B            0.01       0.10                                                  W            0.0        1.0                                                   Y            0.0        0.1                                                   ______________________________________                                    

said composition having been cooled at a rate of approximately 600° F.per minute or less.
 2. The composition of claim 1, which has been cooledat a rate between 50° and 600° F. per minute.
 3. As a composition ofmatter an alloy consisting essentially of the following ingredients inthe following proportions:

    ______________________________________                                                    Concentration in Weight %                                         Ingredient  Claimed Composition                                               ______________________________________                                        Ni          balance                                                           Co          8                                                                 Cr          10                                                                Mo          4                                                                 Al          4.5                                                               Ti          4.0                                                               Ta          3.0                                                               Nb          1.5                                                               Zr          0.06                                                              C           0.05                                                              B           0.03                                                              V           1.0                                                               ______________________________________                                    

said composition having been cooled at a rate of approximately 600° F.per minute or less.
 4. The composition of claim 3, which has been cooledat a rate between 50° and 600° F. per minute.
 5. As a composition ofmatter an alloy consisting essentially of the following ingredients inthe following proportions:

    ______________________________________                                                    Concentration in Weight %                                         Ingredient  Claimed Composition                                               ______________________________________                                        Ni          balance                                                           Co          8                                                                 Cr          10                                                                Mo          4                                                                 Al          4.5                                                               Ti          4.0                                                               Ta          3.0                                                               Nb          1.5                                                               Zr          0.06                                                              C           0.05                                                              B           0.03                                                              V           1.0                                                               ______________________________________                                    

said composition having been cooled at a rate of approximately 600° F.per minute or less.
 6. The composition of claim 3 which has been cooledat a rate between 50° and 600° F. per minute.